Disease

Blight control

Potato Blight is still the most serious disease of potatoes in Ireland despite recent advances in fungicides, improved varieties, better disease forecasting, and so on. As yet there is still no means of eradicating an outbreak in a crop. Therefore on disease prevention is key. There are two main factors to be considered:

(a) The Source of the disease, and

(b) The Spread of the disease.

Source of Potato Blight

Potato blight is caused by the Phytophthera Infestans, which is an oomycete pathogen. The fungus survives the winter in blighted tubers. These may remain in the soil after the previous crop, or in dumps where potatoes have been discarded after grading. Planting blighted tubers is also a potential source of the disease. Therefore make every effort to ensure that all tubers are picked at harvest time and that volunteer potato plants are not allowed to grow the following year. These plants can act as a source of infection for nearby potato crops. Destroy all potato dumps by applying Glyphosate or Diquat based products, and make sure that only blight free tubers are sown.

Spread of Blight

Blight is commonly recorded for the first time each season in unsprayed maincrop potatoes around mid to late July. Weather conditions which favour the spread of the disease occur when temperatures rise above 10oC and relative humidity rises above 95%. The typical mild, damp Irish summer is ideal for the spread of the disease. The meteorological service issues forecasts of the spread of potato blight whenever the temperature and relative humidity are suitable for the spread of the disease.

For more information on blight please visit 

Other Potato Diseases

Common Scab (Streptomyces scabies)

Caused by a very common soil organism attaching tubers at an early stage of development especially when conditions are dry. Most prevalent on light freely draining soils, and in dry seasons.

Common scab is present in most potato production areas in Ireland. The disease does not effect yield, but it can greatly reduced tuber quality, and symptoms are often significant enough to affect grade. Tuber lesions are generally not vulnerable to secondary storage rots; therefore, the disease does not affect the storability of the crop. The name common scab distinguishes this disease from powdery scab, which is less prevalent. Scab on rooster is the heartbreak of farmers growing for the washing trade.

Prevention in the Field: Management of Common scab can be difficult. Disease severity varies among fields, and the reasons for the variation are often not apparent from soil tests or previous agronomic practices. The disease is usually more severe during warm, dry seasons (with soil temperatures above 22° C), which appear to favour the growth of the pathogens and the production of thaxtomins. Under these conditions, disease severity may be high, even in resistant potato varieties. Prevention depends on a combination of practices.

1. Avoid planting scabby seed tubers. Streptomyces strains vary in virulence, and planting scabby tubers may result in reinfesting the field with another, more aggressive strain.
2. Increase the time between successive potato crops. A 3 to 4 year rotation is helpful. This reduces scab incidence to a relatively constant level, but it seldom completely eliminates soil populations of the pathogen.
3. Varietal resistance or susceptiblity influences but does not determine scab incidence, and lesion type. Potato varieties vary considerably in scab resistance, but none are immune.
4. Maintain high soil moisture (approaching field capacity) for 4 – 6 weeks, beginning at tuber initiation (swelling of the stolon tips).
5. Avoid application of fresh lime.

What To Do: Each successive layer of new tissue on a tuber is susceptible to attack for a period of ten to fifteen days only, and it then becomes immune for the rest of the season. As the tuber grows fresh areas of new tissue at the rose end are vulnerable, but the pattern of growth is such that most of the transition from stomata to lenticles is completed during the first six weeks after tuber initiation. The major concern, therefore, is to control common scab during the initial six weeks. Irrigation prevents infection by the common scab organism. So growers using irrigation to control common scab must be prepared for a 6 weeks irrigation programme.

When irrigating to control common scab note the following:

1. Identify the crop where skin finish is a priority.
2. Start irrigation in time – i.e. at tuber initiation – this stage must be identified by lifting plants and examining stolens (physical ageing will reduce the period from planting to tuber initiation.
3. Apply small amounts of water – 12 / 15 mm
4. Depending on soil moisture, applications may have to be made every 7 days

Using Sulphur to lower soil pH instead of using irrigation is not the road to go.

Rubbery Rot (Geotichum candidum or Oospora lactis)

Symptoms are similar to pink rot in every respect. Differentiating between this disease and pink rot requires laboratory facilities.

• Caused by soil borne fungus of Getrichum spp. Infected tubers have regular brown patches with black margins. Affected tubers smell "sweet", their flesh has a rubbery texture and leaks when stored under warm conditions. Cut tubers turn dirty pink colour after three to four hours at 20ºC.
• Outbreaks increase after warm weather in early Autumn in areas of high rainfall and / or irrigation and most particularly in poorly drained soils. The variety Maris Piper is particularly susceptible. Fields where soil compaction has been a problem and a wet autumn /winter are a high risk - e.g. where potatoes have followed sugar beet.
• Infection may be secondary following, for example, pink rot. It can also trigger secondary bacterial breakdown in store.

Pink Rot

This disease was first recorded in 1913 in the West of Ireland where it was called "water rot" or "water slain" and is now worldwide in distribution. In the British Isles, although it varies from one year to the next in severity, past records show that pink rot levels have been high in years where late blight has been absent, and that it is most prevalent in hot, dry years in heavy soils that retain adequate moisture for infection. It is a disease that can sometimes cause serious losses in particular crops but is so markedly seasonal and often so very local that its general importance is comparatively small. It is usually observed at harvest and, during early storage, and is a major cause of pockets of wet rotting tubers in potato piles in poorly ventilated stores.

Symptoms: It is caused by a soil borne fungus of the phytophthora species. Affected tubers are rubbery in texture exuding juice where pressed and often may be confused with tubers exposed to frost or prolonged chilling. The disease usually starts at the stolon end, the affected skin becoming discoloured and darkened around the lenticels. On cutting an infected tuber, the diseased area is rubbery in texture, exudes water and is slightly off-white but on exposure to air turns pink within 30 minutes.

The colour turns from pink to purple and eventually a dark brown. It has a characteristic smell of vinegar.

Control Measures:

• Severely affected crops should be sold off the field.
• 5 year rotation should help to reduce soil carry-over.
• Ventilate as much as possible to cause diseased tubers to mummify and thus reduce the development of soft rot pockets.
• Foliar application of metalaxly does not give adequate control of the disease.

Brown Rot of Potatoes (Ralstonia solanacearum)

Why the Concern?
This is a serious non-established bacterial disease of potatoes, and is a quarantine disease listed in the EC Plant Health Directive and is a notifiable disease. The major yield loss caused by brown rot is through rotting of tubers. Brown rot is the main limiting factor in potato production in many parts of the world. If it were to become established in the Ireland, not only would our potato industry suffer direct yield losses, the knock-on effect on the nation's seed-potato industry could be substantial. Once established, the costs of control could also be very high. Control of this disease requires vigilance from all sectors of the industry, from growers through to merchants, packers and retailers.

Symptoms of potato brown rot
Brown rot is caused by the bacterium Ralstonia solanacearum and is widely distributed in warm tempera te areas of the world. In the EC it has recently been reported from Belgium, France, Germany, Greece, Italy, Spain, The Netherlands and the UK.

The bacterium can cause wilting of the potato plant but the symptoms you are most likely to see are in the tuber.

What is being done to help?

• Legislation

  Under the terms of the EC Plant Health Directive and the Plant Health, importation of material carrying these diseases is prohibited. In addition, specific control directives for each disease lay down measures aimed at preventing their spread and, if possible, eradicating them.

• Surveys

  The Department of Agriculture Food and Rural Development carry out annual surveys in Ireland.

  Plant Health and Seeds Inspectors carry out an annual survey of seed and ware potato stocks, including those grown from both home and non-Irish seed.

What can you do?

Plant only classified seed All classified seed potatoes produced in the EC must have been derived from material found free from these diseases.

Control groundkeepers Potato groundkeepers are a key factor in the long-term survival of both diseases. Their control removes an important source of disease inoculum.

Good hygiene Regularly clean and disinfect all machinery, equipment, containers, vehicles and storage facilities used during potato production.

Don't dump waste on agricultural land Discarded potatoes and waste from potato processing could harbour these diseases and should not be dumped back onto fields. If possible, dispose of all potato waste at an approved tip or by incineration.

Keep a good look out

If you see any of these symptoms you should notify Teagasc or The Department of Agriculture

There have been no known outbreaks of the disease in Ireland

There have been five outbreaks of Potato Brown Rot in potato crops in the UK: two in the Thames valley (in 1992 and 1995), two in Northamptonshire (in 1999) and one in Kent (in 2000). The bacterium has also caused two outbreaks of disease in tomato crops grown at one locality in Bedfordshire (in 1997 and 1998).

How the disease is spread In the UK outbreaks in both potatoes and tomatoes were linked to irrigation of crops from contaminated rivers where the bacterium persists and multiplies by infecting wild plants of Woody nightshade (Solanum dulcamara; also known as bittersweet which have roots in the water.

Seed and flower of Solanum dulcamara

BROWN ROT IN POTATOES

Brown Rot is to potatoes what foot and mouth is to livestock. It spread from Europe to the UK in 1992 and since then it has spread to a number of locations through the country including Scotland. So far Ireland is free, but with free trade in potatoes for how long more can we remain so?

The disease (Ralstonia Solanacaerum) causes a wilt in the growing crop and a rot in the tubers. It is caused by a bacterium and its host is a weed called Woody Nightshade or Bittersweet (Solanum Dulcamara). This weed is to be found in Ireland already. The weed is also common in the UK and a major problem there was where the weed was growing with its roots in water, it passed the disease into the water which was then used to irrigate potato crops. This caused the disease to spread steadily.

Watch Out

Despite great efforts by Ministry of Agriculture officials, the disease has spread from the Thames outside London to the Tay in Scotland. They have now banned irrigation from several rivers and are using glyphosate to kill the weed wherever they can.

Our Department of Agriculture are very keen to keep the disease out of this country. While they do their part the one thing we can do is watch out for the weed and if we find it inform someone in the potato world about its location. The weed grows about 8 inches tall and will grow almost anywhere. It flowers from June to September and they are like those of the potato, purple with a bright yellow centre. It is a straggling type of weed, woody at the base, hence the name Woody Nightshade.

In the Autumn it produces red berries, oval in shape, about ½ inch in length. We have no idea what its distribution is throughout Ireland nowadays. We are anxious to learn. If you come across it, leave it growing but inform your local Teagasc Office or your local Potato Inspector.

Ring Rot

Ring Rot is an extremely serious and highly infectious potato disease with the potential to seriously decrease ware yields and jeopardise our seed potato industry. Up to November 2003, the disease had never been recorded in the British Isles.

Ring rot has been described as the equivalent of Foot and Mouth disease in the plant world. It is a bacterial disease, which is caused by the bacterium Clavibacter michiganense subsp. Sepedonicus. Prior to the recent outbreak in Wales it has never before been found in these islands and it has still never been found in Ireland. It is, however, widespread in many countries, especially in North America, but it does occur in some EU countries.

Quarantine Disease: Since all EC Plant Health Authorities are vigilant in looking for symptoms in tubers it is likely that spread between member states would occur in cultivars that carry infection latently. An infected stock could be propagated and distributed within Ireland for several years before expressing symptoms. Once infection becomes established eradication would be very difficult as is testified by the persistence of C. michamense sepedonicus in Canada and Denmark, despite rigorous efforts at eradication.

The response to initial detection would probably be instigation of an eradication programme involving destruction of the infected crop and all other stocks that had a common origin with the infected crop during the previous two years. On affected farms all equipment and stores would have to be disinfected and potato production halted for a number of years.

A comprehensive programme of laboratory testing to screen for latent infection would have to be established together with an administrative unit to organise and implement the statutory aspects of the eradication programme. Furthermore, since ring rot is a quarantine disease, other member states could ban imports of seed potatoes from Ireland.

Ireland: The Department of Agriculture here is aware of the discovery of Ring Rot in Wales and states that the disease was also found recently in Denmark, Finland, France, Germany, Greece, Spain and the Netherlands. The Department emphasised the fact that Ireland has never had a case of Ring Rot but there are contingency plans in place in the event of an outbreak.

The Department carries out a survey each year for the Ring Rot and Brown Rot organisms. Approximately 500 samples (each consisting of 200 tubers) are taken from national production and examined in the laboratory. Additional samples are taken from potato tubers imported into Ireland and these are also tested for the two organisms. In addition, routine and targeted sampling is carried out along watercourses, and these samples are analysed for Brown Rot.

Ireland is a high-grade seed status area. Imports from Wales, or other areas, which have not got high-grade status, are subject to checks. The Department contacts the country of origin to find out if checks have been carried out on the consignment for these diseases. If proof of disease-free status of a consignment is not available then sampling and testing is carried out by Department staff. Where this happens, the consignment is held in quarantine until results of these tests have proved negative.

Disease Spread: Ring Rot bacteria survive between seasons mainly in infected seed tubers. They are also capable of surviving for two to five years in dried slime on surfaces of boxes, bins, sacks, etc. or harvesting and grading machinery. The organism can even withstand temperatures well below freezing. Cool, dry conditions favour longer survival.

The Ring Rot bacteria need potato debris in the soil to survive – no debris means no survival. However, it will survive from season to season in volunteer potatoes, so, like with blight, good land and area hygiene is very important to help control.

The pathogen is highly infectious and can easily transmit from diseased tubers to healthy tubers. But some of avenue of penetration is needed for infection and surface wounds are the main source. In countries where the tubers are sliced to make seed this operation is very important in the spread of the disease through the seed.

So care at handling is very important. The bacteria may also be spread from infected to “clean” tubers during planting, particularly if using planters where the tuber cups may damage the tuber seed surface.

The bacteria multiply within the plant following infection and are then moved throughout the plant via the water-conducing tissues. From here they move to the young developing tubers.

In the world context, there are varieties of potatoes that are quite resistant to Ring Rot and rarely, if ever, show symptoms of the disease when infected. This means there is a risk of symptom-less infections occurring in these cultivars, and they have been shown to act as carries of the disease. Russet Burbank is regarded as partially resistant and does not always show symptoms of the disease during the first year of infection.

Symptoms: The main symptom is in the tuber but Ring Rot can cause a wilting effect in growing potatoes. Severe Ring Rot can result in wilting of leaves and stems along with yellowing and death of leaves.

However, it is not uncommon that the foliage would show no infection symptoms. Ring Rot gets its name from the characteristic breakdown of the vascular ring within the tuber.

This often appears as a creamy-yellow to light-brown, cheesy rot. The symptom is most obvious when a diseased tuber is cut crosswise at the stem end. In severe cases, the vascular ring may be separated, and a creamy or cheesy exudate can be forced out from this tissue when the tuber is squeezed.

On the outer surface, severely diseased tubers may show slightly sunken, dry, cracked areas. Infected tubers are often invaded by secondary decay organisms, which lead to complete breakdown. Symptoms of the disease effect on the vascular tissue of the infected tubers are often less than clear. Sometimes only a sporadically appearing dark line can be seen, or only a continuous, yellowish discoloration.

Because of this, laboratory tests should always be performed to confirm a diagnosis.

Silver Scurf

Silver scurf is a serious disease problem for potato growers supplying the washed potato trade. Rooster, our main variety for washing, is very prone to the disease, as is all red skinned varieties. Each year, our supermarkets require more and more of their potatoes to be washed, so growers are now forced to control silver scurf by whatever means they can.

Silver scurf is a disease of potato tubers which is caused by the fungus Helminthosporium solani. The disease has been present in Ireland for almost 100 years but has been of little interest or importance to potato producers.

The symptoms of silver scurf consist of silver coloured patches on the surface of the tuber. The fungus gains entry to the tuber via the lenticels. Infection is confined to the corky cells and the silvery appearance results from air pockets being formed between the infected cells. These unsightly patches on the tubers can lead to washed samples being unmarketable. The disease is not usually evident at harvest time but increases during storage and is most obvious on coloured skin varieties and on damp or washed tubers. The disease can develop rapidly under storage conditions of high temperature and humidity and under these conditions the fungus can also sporulate and lead to a black or sooty deposit on the tuber. In severe cases of infection the skin may flake off and the tubers may become dehydrated and shrivelled.

Seed is the Source: Research in other counties would indicate that with the exception of the potato tuber the disease affects no other part of the plant. This research would also indicate that the seed tuber is the most important source of inoculum. The role of contaminated soil, if any, in spreading the infection is not clear but would appear to be less important than the seed tuber.

Within a few days of planting, spores, produced at the edge of silver scurf lesions on seed tubers, are released into the soil. Increasing numbers of spores are produced during the growing season as lesions enlarge. Therefore, seed with moderate infections can produce a greater number and give more silver scurf than seed with severe infections which are older and have lost the ability to sporulate.

First infections on young tubers are found in July but disease symptoms are not usually seen until late August or September, initially as discrete lesions which later coalesce as they enlarge. The disease increases in severity during storage, but as it is often already present at harvest, fungicide applied to ware tubers during loading into store may apparently fail to control it. The treatment should prevent enlargement of lesions, and hence further infection.

Treating diseased seed tubers with fungicides prevents sporulation on their surface and will give healthy crops. This indicates that chemicals have to be deposited over the whole of the tuber surface. Seed can be treated at any time during the winter.

Rhizoctonia

Rhizoctonia disease of potato, often referred to as Black Scurf, is caused by the fungus Rhizoctonia solani Kuhn. The disease is found in most potato producing areas of the world. It is was reported in Maine in 1913 by Morse and Shapovalo.

The disease cycle is very straightforward. Inoculum usually is introduced into fields on seed tubers, although it may be introduced via contaminated soil.

The management of Rhizoctonia disease requires an intergrated approach and knowledge of each stage of the disease. One of the most important initial management decisions that should be considered by potato growers is to purchase and plant only high quality seed potatoes that are not infected with black sclerotia of the Rhizoctonia fungus. If black sclerotia are present on seed they should cover no more than 5% of the potato surface.

Poor stands, stunted plants, reduced tuber number and size, and misshapen tubers are characteristic of the Rhizoctonia disease.

Role of Infected Seed: Potatoes plants affected are characterised by a lack of vigor because much of their energy has been used to produce secondary or tertiary sprouts before a plant emerges. Sometimes, heavily infected seed will not produce an above ground plant. Instead, it will produce a stolon with several small tubers. This symptom is referred to as “no top” and can be confused with the same symptom caused by physiologically old seed that has been desprouted, i.e. “little potato”.

Stolons and roots, like sprouts, can be killed by the pathogen. The root system is reduced when this occurs. The number, shape and size of tubers produced are also affected when the roots and stolons are attacked.

Tuber symptoms include necrosis in stem end, russeting, cracking, knobbiness, infected lenticels and malformed tubers. Seed tubers infested with sclerotia and mycelium are major sources of inoculum for future crops.

Rhizoctonia disease can be airbourne, seedborne or soilborne. Seedborne inoculum, is present on tubers when they are placed in storage. When contaminated seed tubers are planted in the soil, the fungus is in close proximity to developing sprouts, stolons, and roots, and infection of these tissues can occur when they are young and highly susceptible. If sprouts are killed, plants may fail to emerge, or emergence may be delayed. This may lead to gaps in crops and reduced yields, especially in light soils and cold wet springs.

STEM CANKER: The fungus attacks underground sprouts (fig. 2) before they emerge from the soil. Stolons that grow later in the season can also be attacked (fig. 3). The damage varies. The fungal lesion, or canker, can be limited to a superficial brown area that has no discernible effect on plant growth. Severe lesions are large and sunken, as well as necrotic. They interfere with the normal functioning of stems and stolons in translocating starch from leaves to storage in tubers. If the fungal lesion expands quickly, relative to growth of the plant, the stolon or stem can be girdled and killed.

Damage is most severe at cold temperatures when emergence and growth of stems and stolons from the tuber are slow relative to growth of the pathogen. Wet soils also contribute to damage because they warm up more slowly than dry soils and excessive soil moisture slows plant development and favors fungal growth. If Rhizoctonia damage is severe and lesions partially or completely girdle the shoots, sprouts may be stunted or not emerge above the soil. Stolon cankers reduce tuber numbers and size and are identical to shoot cankers in appearance.

Poor stands may be mistaken for seed tuber decay, caused by Fusarium species or soft rot bacteria, unless the plants are excavated and examined. Rhizoctonia does not cause seed decaly; its damage is limited to sprouts and stolons. Poor stands and stunted plants can also be caused by blackleg, a bacterial disease that initiates from the seed tuber and progresses up the stems, causing a wet, sometimes slimy, rot. In contrast, Rhizoctonia lesions are always dry and usually sunken.

Aerial Tubers: In Ireland, usually in July, curling of the top leaves on many plants in the field, is always a sure sign of the presence of Rhizoctonia.

The interference of carbohydrate movement may cause the formation of aerial tubers in the leaf axils of stems and the formation of many small, misshapen tubers. These affected plants are more erect and frequently are more difficult to kill with desiccants. This symptom can be observed after dessication when individual plants are standing erect in an open field where most of the plants are dead.

Late season damage to plants is a direct result of cankers on stolons and stems causing problems with starch translocation. Tubers forming on diseased stolons may be deformed. If stolons and underground stems are severely infected with Rhizoctonia canker, they cannot carry the starch produced in the leaves to the developing tubers. In this case, small green tubers, called aerial tubers, may form on the stem above the soil (fig. 4). Formation of aerial tubers may indicate that the plant has no tubers of marketable quality below ground.

White Mold: At the end of the growing season, the fungus produces its sexual state, Thametephorus cucumeris, on stems just above the soil line. It appears as a superficial delicate white mat which is easily removed (fig. 5 and 6).

The pathogen overwinters as sclerotia and mycelium on infected tubers, in plant residue, or in infected soils. High soil moisture, cool temperatures, high soil fertility and a neutral to acid soil (ph of 7 or less) favor development of the disease. A whitish mold can be formed on stems just above the soil line. Its role in the disease complex is not well known. When it is observed, stem lesions are often severe.

Black Scurf: The phase of the disease called black scurf is common on tubers produced commercially and in home gardens. The irregular, black to brown hard masses on the surface of the tubers are sclerotia, or resting bodies, of the fungus (fig. 1). Although these structures adhere tightly to the tuber skin, they are superficial and do not cuase damage, even in storage. They do perpetuate the disease and inhibit the establishment of a plant from the tuber if it is used as seed.

Black scurf is the most noticeable sign of Rhizoctonia. But the most damaging phase of the disease occurs underground and often goes unnoticed.

The most conspicuous sign of Rhizoctonia disease is the dark brown to black sclerotia that develop on the tuber surface. This is known as black scurf. Sclerotia do not penetrate or damage the tuber, but they adhere tightly to the skin and resemble soil that will not wash off. Tuber symptoms include malformation, cracking, and in some cases surface blemishes.

MANAGEMENT: Some potato varieties appear more resistant than others to R. solani infection, but there is no evidence of high levels of resistance in any variety in Ireland In the presence of inoculum, minimising the time between planting and sprout emergence can further reduce disease levels. Fungicides applied to seed, are used to protect young sprouts and to reduce disease.

Research has identified that only three of the 13 AG groups of Rhizoctonia known to exist globally cause black scurf and stem canker in the U.K. Ireland is likely to have the same status. AG 3 is the most prevalent (92%) with AG2 (7%) and AG5 <1%) being less common.

Bayer says Monceren is highly active against AG3 and AG2 strains indicating at least 99% of disease causing infections will be covered.

Certis says Rhino has activity against AG5 in addition. To-date Rhino has not been registered in Ireland, perhaps it may be in 2005.

If soil temperatures are below 8C, seed should be planted within 4-5 cm of the soil surface. This will minimise the time between planting and emergence and therefore may minimise sprout infection. However, this practice may not influence root and stolon infection and will not reduce the formation of sclerotia on tubers.

Crop rotation will reduce soilborne populations of R. solani. A four-year rotation is recommended. The fungus Rhizoctonia solani is able to exist freely in soils for seven years and perhaps longer. The principle weed host is scutch grass. The black sclerotia (fungal resting bodies) present on infected seed tubers usually act as the more important source of disease infection. Similarly, Silver Scurf (Helminthoporium solani) and Skin Spot (Polyscytalum pustulans) are predominantly tuber-borne.

CONTROL: There are five products on sale for the control of Rhizoctonia.

1. Monceren IM is a dry powder seed treatment containing pencycuron 12.5% w/w and imazalil 0.6% w/w, recommended for use on potato seed tubers for the control of Black Scurf (Rhizoctonia solani) and a reduction of Silver Scurf (Helminthosporium solani), Stem Canker (Rhizoctonia solani) and Skin Spot (Polyscytalum pustulans) on daughter tubers. Monceren IM acts primarily against tuber-borne infections of Rhizoctonia and Silver Scurf, although useful control of Rhizoctonia arising from the soil-borne infections can also be expected.

   Application: Monceren IM is applied to seed tubers during the planting process with automatic potato planters. It is essential to obtain an even distribution of the product on each seed tuber for optimum efficacy. The action of the potato planter achieves this very efficiently provided Monceren IM is initially distributed to some extent within the hopper-load of seed tubers.

   Measure the quantity of Monceren IM required to treat a hopper-load of seed tubers. One quarter fill the hopper with seed tubers, spinkle one third of the required quantity of Monceren IM over the seed tubers then add a further quarter of the seed tubers and sprinkle a further third of Monceren IM over these tubers, and so on, in layers until the hopper is filled with seed tubers. Thus three layers of Monceren IM have been sandwiched between four layers of seed tubers. depending on the size and shape of the hopper, the number of layers required may be increased, but even coverage of Monceren IM on the tubers is essential when they are planted.

2. Rizolex (tolclofos-methyl)
3. Rovral-flow (iprodione)

4. Rhino (flutolanil)

   This is a liquid product and is registered in the U.K. To-date (2005) it has not been registered in Ireland.

   The application rate as a seed dressing is 200 m per tonne. It costs £17.50 (sterling) per tone.

   The U.K. supplier is Certis.

5. Amistar (Syngenta)

   Amistar has now been registered in Ireland (March 2006). A liquid, it can be sprayed overall onto the soil before planting at 6l/ha., or down the spout at 3l/ha. Applicators for attachment to planters cost €4,500 approx. and are available from Potato Equipment Ireland Limited, Navan, Co. Meath (Johnny McGuinness 087 2596777).

Black Leg

Spotting of Queen bags and the soft rotting of Pinks in store are two problems well known to potato farmers and much feared by them. Both are caused by Blackleg (Erwinia spp.) and can cause near financial ruin in a wet season. Blackleg is always with us but in a dry season we hear very little about it.

As Blackleg does not survive in soil, under normal rotations the main source of the blackleg bacteria is the seed you use to grow a crop. If sufficent Blackleg is present in the lenticels of seed tubers, the wound periderm beneath the lenticel is breached and the mother tuber rots. If this happens soon after planting, sprouts may fail to establish and blanking occurs in the field. More normally it occurs after emergence and the growing crop has estblished. In this situation, the Blackleg bacteria move up the vaccular tissue of the stems and block water transport to the foliage. The leaves turn yellow and the typical black slimy lesion develops above and below soil level.

Blackleg bacteria from the rotting stem, but particularly from the mother tuber, spread through the soil to the developing daughter tubers. These become contaminated on the surface and in the lenticels. The spread is greatest when soil conditions are wet. In very wet conditions or prolonged irrigation, the lenticels of the daughter tubers may extrude and facilitate entry of the bacteria. Under dry soil conditions in the period to harvest, there will be little spread to the daughter tubers. Thus the weather conditions at and before harvest influence the degree of daughter tuber contamination.

Spread at harvest time

Another important way contamination can occur is during harvest when the contents of rotten mother tubers spread to daughter tubers. Blackleg can spread to wound and lenticels.

Once lifted, the survival of Blackleg on the tubers depends on the storage conditions. Where tubers are dried rapidly and kept dry using positive ventilation, the contamination can decrease over a long period of storage but will probably not be elminated. If drying is slow or condensation occurs, the contamination can increase. If it increases very rapidly the wound periderm or barrier below wounds or lenticels may be breached and rooting is initiated. Other factors such as dry matter content can affect the extent of rotting after harvest. Cooling tubers immediately after harvest using refrigeration may prevent the development of rots but not reduce contamination much.

In Ireland, we have very few forced ventilation units. Units such as the Letterbox system, the Aspire or Boxer systems are very scarce.

Spread at storage time

in storage, proper aeration combined with 95% relative humidity and a temperature of 10-13 degree C for 10-14 days favors the healing of wounds. After this period, tubers should be stored at temperatures below 10 degrees C. Good ventilation should be maintained during the storage period to prevent oxygen depletion and moisture condensation without excessive loss of tuber weight due to shrinkage.

Control

1. Reduced generations
2. Spore numbers can rise rapidly after sprouting so during sprouting avoid condensation and sprout damage
3. Early lifting
4. Rapid drying after harvest kills bacteria on tuber surface
5. Keeping dry throughout storage leads to a decline in contamination

What can ware growers do?

• Avoid sprouts developing on seed that can be knocked off at planting.
• Avoid waterlogging when irrigating
• Do not pulverise the haulm when blackleg is present and the crop is wet or rain is imminent
• Minimise damage at harvest
• Avoid lifting crops in the wet
• Dry tubers as rapidly as possible using positive ventilation and maintain dry
• Avoid condensation during storage especially when storage temperatures are 6 degrees C or above
• Clean stores, boxes, trays and machinery between seasons or more frequently if appropiate

Black Dot

Black dot, unlike many other skin blemish diseases, shows distinct variety trends; the worst varieties succumbing to 65% infection and the least affected, a 20% infection rate.

This is the finding of a major Independent Variety Trials programme conducted by NIAB and funded by BPC.

A UK survey between 1989 and 1990 showed infection in 75% of all crops surveyed. Despite being common, little is known about the epidemiology of black dot and how to control the disease.

Black dot can develop on stems, stolons and roots. The organism, Colletotrichum coccodes, produces silvery lesions on the tuber surface - characterised by black micro-sclerotia. Pre-packers will reject samples over a certain threshold.

A significant finding of this survey was that minimising the period between burn-off and harvest can significantly reduce black dot infection.

Research found a link between black dot symptoms on stems and stolons with the final skin infection for Estima in three consecutive years. However, the same method applied to Maris Piper and Saxon gave mixed results.

Even though Black dot infection can be widespread, the symptoms are often not seen until the haulms started to senesce. So test digging is essential.

Source: NIAB (1-9 value: 9 being most resistant)

Sclerotinia or White Mold

Also called Stalk Break, it was found in two potato crops in 1999 and one in 2003. These are the only known outbreaks in Ireland in recent years.

The disease was first noticed and recorded on potatoes in 1837 by Ambrose Balfe - and that was in Ireland! In 1910 it was noted that "next to Blight, this disease is probably the most serious one in the West of Ireland".

It affects haulms and tubers. White fungal growth appears on stalks, usually near ground leve,l from early July. Snow white oval or spherical cushions of mycelium form on the inside and outside of the haulm. Leaves of infected plants become limp and wilt and can also turn yellow. Stems can break at this point. With time the sclerotia turn pink, brown and eventually black. The fungal attack hollows the pith of the stem, which if cut will show pea sized black sclerotia. This is the "tell-tail" sign for this disease.

Tubers: Tubers near the soil surface may be infected, but this phase of the disease is seldom seen. Tuber lesions are initially small, depressed areas developing near eyes. As the lesions enlarge, the flesh shrinks and become superficially blackened, and a watery soft rot develops. Diseased tissues later develop internal cavities filled with mycelium and sclerotia.

Contributory Factors: White mold, also called Sclerotinia stalk rot, has gained importance as a disease of potatoes with the increased use of irrigation and maintenance of high fertility levels. These two cultural practices promote lush, dense canopies, long periods of high relative humidity in the canopy, free moisture on the foliage, and relatively stable temperatures. All of these environmental factors promote disease development.

Control: Apart from a long rotation, there is very little the farmer can do to prevent it.

Management of the canopy environment or irrigation practices to reduce the duration of leaf wetness may help to reduce white mold in some production areas. For example, orienting rows parallel with the direction of the prevailing winds or planting cultivars with smaller or upright vine growth may be of some value in fields with a history of severe disease. An alternative is to schedule irrigation to allow the foliage to dry before nightfall.

Internal Rust Spot

This is a major problem in Golden Wonder in Ireland.

Internal Rust Spot is characterised by small, round or irregularly shaped, light tan, reddish brown, or rust-coloured spots or blotches scattered throughout the tuber flesh. The lesions can occur anywhere within the tuber but are generally more common in tissue inside the vascular ring. Affected tubers normally do not show external symptoms, although in some reports internal rust tubers that were malformed or large or had a thickened or abnormally russeted skin.

Internal rust spot may occur at various times throughout the growing season. It may develop soon after tuber initiation or during periods of rapid or uneven tuber growth. In most cases, the disorder occurs toward the eye (rose) end of the tuber, indicating development in midseason or later. Symptoms tend to increase in severity during the growing season and continue to intensify in storage. Symptoms may also first appear in storage. The severity of the problem may increase with time in storage, depending on the maturity of the tubers at harvest and storage temperatures.

Growing Season: The key factors are irrigation and soil that is free from free-living nematodes. However there is no scientific evidence that there is a linkage between high numbers of free living nematodes and internal rust spot. Sprang (a virus) is spread by nematodes and can often be misdiagnosed ad internal rust spot. Fields can be tested for the presence of free-living nematodes (soil test) but even these nematodes are controlled this is not a guarantee that internal rust spot will not occur. and if they are found, a variety like Golden Wonder should not be grown.

Irrigation is the key to success. Deep heavy soils, such as in Co. Meath, are a good help.

Re-absorption of water or other tuber constituents to support vegetative development is associated with the occurrence of internal rust spot. This disorder has also been associated with low or fluctuating soil moisture.

Localised calcium deficiency in rapidly growing portions of tubers has been shown to be associated with internal rust spot. Calcium concentrations have been reported to be lower in tubers with this disorder than in unaffected tubers. Some cultivars have been shown to be more susceptible to internal rust spot than others. Golden Wonder is among the cultivars in which the disorder develops when the calcium level in tubers is too low. A slowing of growth and subsequent rapid enlargement of tubers as a result of temperature and moisture fluctuations usually trigger this localised deficiency. Under conditions of even tuber growth, internal rust spot is less likely to occur.

However, in years of warm early springs and subsequent vigorous foliage growth that results in high yields and large tubers, internal rust spot frequently appears. The necrotic spots usually appear late in the growing season, but the conditions that cause them to develop may have occurred earlier.

Symptoms of calcium deficiency in foliage rarely appears under most field conditions.

Internal rust spot has not been consistently controlled by field applications of calcium, but the lack of success may be due to poor uptake of calcium and movement of it to the tubers, especially during periods of stress, in which the disorder is induced.

The first job is for farmers to get Calcium into the root zone at planting time. This can be applied as gypsum or calcium nitrate. The second job is to have irrigation equipment available, so that if a drought period occurs, it is possible to get the calcium into solution so that it can be absorbed by the roots, and into the tubers.

Management of Internal Rust Spot:

1. Grow cultivars that have shown some resistance. Cultivars differ considerably in susceptibility.
2. Manage irrigation and fertilisation to promote even growth without stress on the plants and tubers (for example, stress due to applications of large quantities of nitrogen fertilisation).
3. Don’t allow regrowth after dessication.
4. Harvest the crop as soon as possible after burning off.

Calcium Nitrate: One U.K. Company, Hydea, are supplying Calcium Nitrate into Ireland for the control of Internal Rust Spot. No research has been carried out on this product in Ireland, but if using it, take care in relation to nitrogen application rates.

Recommendations: Apply 60 – 70% of the recommended nitrogen requirement in the base dressing. Top dress the remainder as Calcium Nitrate Tropicote at tuber initiation 3 –4 bags/acare (375 – 500 kg/ha).

For further information contact: Tom DeCamp, Hydro Agri(UK,) Ltd., Immingham, N.E. Lincolshire DN40 2NS
Telephone: 07831 282096
Email: tom.decamp@hydro.com
www.hydroagri.couk

Test for Free-Living Nematodes This test can be carried out by A.D.A.S. at High Mowthorpe and is of interest to Golden Wonder growers concerned about internal rust spot.

The address is as follows:
Mr. Steve Ellis,
A.D.A.S., High Mowthorpe Research Centre, Duggleby, North Yorkshire, YO 17 8BP, England

The cost per sample is £60 sterling and it takes about ten days for the results to come back

Dry Rot Fusarium

Most seed lots contain some level of Fusarium dry rot. Infected seed tubers infest the surrounding soil, where the pathogens can survive for several years. Fusarium spp. cannot infect intact tuber periderm or lenticles. However, normal potato production practices result in periderm wounding and therefore the opportunity for infection. Fusarium spp. are often spread during planting operations and can spread whenever seed tubers are handled, because wounding often occurs during these mechanised operations. Large numbers of spores are liberated from seed tubers infected with dry rot; these spores can easily infect bruised surfaces of seed.

Harvesting the Seed: Dry rot pathogens are spread during harvest. Soil borne inoculum is present on the surface of harvested tubers and contaminated equipment; infection occurs through wounds caused by mechanised harvesting and handling.

Certain cultivars are tolerant. Tubers are most tolerant when harvested; susceptibility increases during storage, reaching its maximum in early spring, about planting time. Seed lots infected with Potato virus X are more resistant that those free from the virus when harvested.

Avoiding injuries to tubers and providing conditions that promote wound healing are the most important management practices, because Fusarium spp. can infect potatoes only through wounds.

1. Tubers should be harvested from desiccated haulums. Avoid harvesting at temperatures below 7◦C, cold tubers are most susceptible to bruising injury. Adjust equipment to minimise impact during harvest and subsequent tuber handling.
2. Provide conditions for rapid wound healing of harvest injuries early in storage: includes high humidity, good aeration, and temperatures of 13 -18◦C for 14 – 21 days
3. Post harvest application of fungicides can provide disease control.

Storage: In storage, dry rot develops most rapidly at high relative humidity and temperatures of 15 - 20◦C. Lower humidity and temperature retard infection and disease development. However, dry rot continues to develop even at the lowest temperatures safe for storing potatoes. This is of great importance in Ireland where much of our seed is fridge stored. Avoiding mechanical damage at harvesting time and when loading into store is vital. Applying a seed fungicide to seed going into store and again on its way out is a major plus in reducing dry rot.

Planting: If the soil moisture and temperature is suitable for rapid sprout growth and emergence, seed tuber decay after planting may be of little consequence. Conditioning seed tubers, from cold storage, at 10 - 15◦C for 1 week reduces bruising and the risk of decay, accelerates sprout growth, and promotes would healing. However, losses are increased if contaminated seed is held for several days or weeks before planting, or if it is planted in soil that is too cold or too dry for prompt sprout emergence. If the soil is excessively wet after planting, secondary infection by soft rot bacteria is increased.

1. Some level of Fusarium dry rot is almost always present in commercially available seed, but growers should avoid heavily infected seed lots.
2. Seed treatment will not cure dry rot, but will reduce the incidence of new infection while the wound periderm develops.
3. Plant when seed and soil temperature are with 5◦C of each other, and plant in soil sufficiently moist to promote quick emergence and good wound healing. Encourage quick emergence by shallow planting if appropriate.